As a shape of terminal formed on an IC (Integrated Circuit), a pin-type or lead-type terminal and a ball-type or bump-type terminal are known. An IC having pin-type terminals arranged in a row is known as a PGA (Pin Grid Array). An IC having ball-type terminals arranged in a row is known as a BGA (Ball Grid Array).
The BGA is more suitable for a high-density mounting than the PGA for the following reasons.
(1) It is possible to narrow a pitch between the terminals. As a result, it is possible to make a small-sized IC which has many ball-type terminals.
(2) It is not necessary to make a hole in a land of a printed circuit board, wherein ball-type terminals are not passed through the hole. As a result, it is possible to make a small-sized land. Further, it is possible to narrow a pitch between the lands.
FIG. 8 shows a side view of a printed circuit board 70 on which the BGA 60 is mounted. The BGA 60 has an array of the ball-type terminals 61. Then, corresponding to the arrays of the ball-type terminals 61, an array of the lands 71 is formed on the printed circuit board 70. Each ball-type terminal 61 is connected with each land 71 by the solder 80.
The BGA, of which terminals are arrayed in a row with a narrower pitch than 0.8 mm, is known as a CSP (Chip Size Package) because a package-size of the BGA is close to a size of a chip inside the BGA package.
However, in a case that a pitch between terminals is about 0.5 mm and the CSP has two arrays respectively arranged in an inner row and in an outer row, it is technically impossible to mount the CSP on a printed circuit board for the following reasons (1).about.(4).
(1) The printed circuit board 70 shown in FIG. 9 is considered in this discussion. On the printed circuit board 70, with a pitch of 0.5 mm, an array of inner lands 72 of a circular shape and an array of outer lands 73, also of a circular shape, are formed. Further, on the printed circuit board 70, a pattern 74 from the inner land 72 is passed through a gap between the outer lands 73.
(2) In the prior art, a diameter of the outer land 73 is 0.4 mm and a diameter of the inner land 72 is also 0.4 mm.
(3) If the width of the pattern 74 is 150 .mu.m, which is generally used, it is impossible for the pattern 74 to pass through the gap between the outer terminals 73 because the gap is 0.1 mm (=0.5-0.4=100 .mu.m).
(4) Further, if the width of the pattern 74 is 50 .mu.m, which is the minimum for practical use, it is also impossible for the pattern 74 to pass through the gap between the outer terminals 73 because a gap between the the pattern 74 and the adjacent outer terminal 73 is typically 25 .mu.m (=(100-50)/2).
On the other hand, it is considered to draw the pattern 74 from the inner land 72 via a thru-hole which is formed in a area 75 surrounded by the array of the inner lands 72. However, in the case of CSP, extremely small thru-holes are necessary for this purpose because the area 75 is very small and many patterns 74 are formed. Since such a small thru-hole cannot be formed by usual multi-layer techniques, the printed circuit board becomes very expensive. Thus, it is not practical to use such a thru-hole.